NMR shifts, orbitals, and M⋯H-X bonding in d ⁸ square planar metal complexes

We have investigated M⋯H-C interactions in d ⁸ square planar Rh and Pt complexes involving different types of C-H bonds, as well as a M⋯H-N hydrogen-bonded complex, using a combination of DFT and "atoms-in-molecules" (AIM) theory. The AIM analysis shows that both M⋯H-C and M⋯H-N interactions in d ⁸ square planar complexes are of a closed-shell, electrostatic nature, similar to protein backbone hydrogen bonds. However, at the shortest M⋯H distances, both interactions have partial covalence. We find no evidence for the involvement of d _z ² orbitals in M⋯H-C interactions but do find evidence for the involvement ofd _xz/yz orbitals in M⋯H-C bonding. The DFT calculations reproduce well the experimental proton NMR chemical shifts, with a theory-versus-experiment correlation coefficient R ² = 0.985. There are large downfield ¹H NMR chemical shift changes on metal bonding, accompanied by changes in shielding tensor orientations.